1. Field of the Invention
The present invention relates to an organic field-effect transistor and a method of manufacturing the same.
2. Discussion of the Background
Generally, in the field of organic electronics, organic field-effect transistors and organic display elements, such as organic emission diodes, are subjects of interest because they may be used in single electronic circuits that may be produced without sophisticated semiconductor fabrication processes. In particular, single circuits based on organic and polymer semiconductors may be used for large-surface displays and transponders.
P-type and n-type semiconducting materials may be defined differently in organic electronics than in inorganic semiconductors, such as silicon, where, generally, a type of dopant defines semiconductor conductivity. In organic semiconductors, however, p-conductivity and n-conductivity relate to the polarity of a high mobility charge carrier. An electron has high mobility in an n-conducting semiconductor, and a defect electron (hole) has high mobility in a p-conducting semiconductor. A dopant having the polarity of excess charge carriers may be used in a conventional organic emission diode, but using such a dopant in an organic semiconductor may be difficult (Zhou et. al, Appl. Lett. 81, pp. 4070, 2002).
In organic field-effect transistors, an organic semiconductor material is disposed between a source electrode and a drain electrode. Applying a sufficient voltage to a gate electrode forms an electric field in a channel region between the source and drain electrodes. As a result, charge carriers (electrons or holes) flow into the channel, thus increasing conductivity in it. The channel may be formed by holes in a p-type transistor or by electrons in an n-type transistor. Therefore, a gate voltage may control transistors.
When counter charges migrate from the gate electrode to the channel, the electric field deteriorates, ultimately resulting in decreased functionality of the transistor structure. In order to prevent this problem, a charge carrier blocking layer (dielectric layer) may be interposed between the channel and the gate electrode.
However, an organic transistor including the charge carrier blocking layer may have a low maximum attainable current. Generally, the maximum attainable current is a critical factor for organic transistors, particularly in active matrix organic light emission devices.
The maximum attainable current depends on the length and width of the conductive channel, which may be composed of an inorganic and semiconducting material, and on the charge carrier mobility in the organic semiconductor.
To obtain the maximum attainable current, the channel should be short, wide and have high charge carrier mobility. However, there is a limit to the structural dimensions. Additionally, a surface capacity Ci of the dielectric should be considered to obtain the maximum attainable current. Also, a high capacity may be required in semiconductors to keep a sufficient density at a low gate voltage (VGS).
The following formula shows a correlation between an attainable current I and important parameters in an organic field-effect transistor's linear operation range:
      I    =                                        C            i                    ·          W                L            ·      μ      ·              (                              V            GS                    -                      V            th                    -                                    V              DS                        2                          )            ·              V        DS              ,where Vth is a transistor threshold voltage, VDS is a voltage applied between a drain contact and a source contact, VGS is a gate voltage, and μ is the charge carrier mobility.
According to this formula, the surface capacity Ci and the charge carrier mobility μ should have large values in order to obtain high maximum attainable current.
When channel thickness decreases, the surface capacity Ci increases. However, reducing the thickness of the charge carrier-blocking layer is particularly limited by hole density and a disruptive discharge voltage.
Additionally, a dielectric with a relatively high dielectric constant may be used to increase a switchable current in organic field-effect transistors (Dimitrakopoulos et al., Science 283(1999), pp. 822). In this case, the dielectric may be composed of a ferro-electric inorganic material, such as barium zirconium titanate. Such commonly used dielectric materials may be coated by sputtering. However, sputtering may require relatively high input energy and temperatures.
There may also be difficulties in using sputtering to form an organic functional layer, such as a channel composed of an organic semiconducting material. While sputtering may be used to form an organic semiconductor and other organic functional layers that may be formed in a final manufacturing process, it may not be possible to use a top gate structure.
FIG. 1 is a sectional view showing a conventional organic field-effect transistor. Referring to FIG. 1, a source electrode 2 and a drain electrode 6 are deposited on a substrate 1, and an organic semiconductor layer 3 is deposited on the source electrode 2 and the drain electrode 6. A charge carrier blocking layer 4 is deposited on the organic semiconductor layer 3, and a gate electrode 5 is deposited on the charge carrier blocking layer 4.
The organic semiconductor layer 3 may be composed of a p-conducting material. When a sufficient voltage is applied to the gate electrode 5, charge carriers 7 (holes) move into a channel formed between the drain electrode 6 and the source electrode 2 in the organic semiconductor layer 3. As a result, the channel may have high conductivity. Therefore, applying a voltage to the gate electrode 5 may control the transistor.
However, hole density and a disruptive discharge voltage may limit the minimum possible thickness of the charge carrier blocking layer 4.
Furthermore, depositing a ferro-electric inorganic material, including barium zirconium titanate (BZT), on the organic semiconductor layer 3 to form the charge carrier blocking layer 4 may require a high temperature with a high energy input. Therefore, directly depositing the ferro-electric inorganic material on the organic semiconductor layer 3 may damage it.
Since a minimum thickness of the charge carrier blocking layer 4 may not be decreased independently, and depositing the charge carrier blocking layer 4 on the organic semiconductor layer 3 may damage it, the maximum attainable current of a conventional organic field-effect transistor and application of the organic field-effect transistor with pre-specified structural dimensions may be very limited.